This invention relates to a method of producing a composite nuclear fuel cladding, and more particularly to a method of producing a zirconium liner lined on the inner surface of a fuel cladding of zirconium alloy covering nuclear fuel pellets.
A fuel cladding used in a nuclear reactor is required to have an excellent corrosion resistance, non-reactivity and excellent thermal conductivity, a small neutron absorption section, etc. Zirconium alloy satisfies the above-mentioned properties, and therefore is widely used for the fuel cladding.
The fuel claddings of zirconium alloy can function very well, however when a great change in reactor load takes place there is the danger that they are subjected to stress corrosion cracking and resultant breakage because of the corrosive action of iodine gas released from the nuclear fuel pellets contained in the fuel cladding and the stresses generated by the expansion of the fuel pellets.
In order to prevent such stress corrosion cracking in fuel cladding, a barrier made of one of various metals is provided between each cladding and the fuel pellets therein. A composite nuclear fuel cladding made of a zirconium alloy and lined with high purity zirconium as a metal barrier is disclosed in Japanese Patent Laid-Open No. 54-59600/1979.
High purity zirconium is used because the pure zirconium liner is capable of remaining more flexible than zirconium alloys during neutron irradiation, and has the effect of reducing local strains produced in the zirconium alloy cladding to prevent stresses and stress corrosion cracking. In order that the zirconium liner functions well, it is preferable for the liner to have the purity of crystal-bar zirconium.
A conventional method of producing crystal-bar zirconium is known in which sponge zirconium is iodinated and then the resultant iodide is subjected to chemical vapor deposition to form zirconium crystal bars. With this method, the reaction speed of the formation of zirconium by the thermal decomposition of zirconium iodide is extremely slow, and is therefore unsuitable for mass-production, so that the cost of the zirconium produced thereby is extremely high.
Another conventional method of producing high purity zirconium is an electron-beam melting method in which a material to be melted can be subjected to a high temperature in a high vacuum atmosphere, whereby impurity elements of a high vapor pressure such as iron are evaporated away. Further, as for a removal mechanism of gases such as oxygen, hydrogen, nitrogen, etc., the hydrogen and nitrogen are separated from the molten metal surface into the vacuum atmosphere in a state of H.sub.2, N.sub.2 while the probability that the oxygen transfers to a gas phase in an atom state is small, the removal of the oxygen is effected based on the phenomenon of volatile deoxidization such that the oxygen is evaporated away in a state of volatile oxides. Namely zirconium can be evaporated away in a form of ZrO. Removal efficiency of iron and oxygen is determined by comparing the vapor pressure of zirconium itself and the vapor pressure of iron, ZrO. The vapor pressure ratios between the zirconium and iron, and between the zirconium and ZrO are given as follows: EQU Fe/Zr=10.sup.5, ZrO/Zr=10.sup.2.
The above relations show that the zirconium can be refined by effectively effecting the electron beam melting.
The electron beam melting method is roughly classified into a rod melting method in which a material to be melted is irradiated with electron beam to drop the molten metal thereby forming a rod-like ingot, and a hearth melting method in which a material to be melted is inserted in a hearth mold and radiated with an electron beam while moving the hearth or an electron gun.
Japanese Patent Publication No. 41-1519 (1966) discloses a hearth melting method; however, it is silent on a hearth mold shape.
Japanese Patent Laid-Open No. 56-67788 also discloses a melting method; however, it can not determine the purity of the material after melting.
In a zirconium liner material, it is thought that a substance causing irradiation hardening is oxygen of impurities contained in metal zirconium. On the other hand, iron contained in the zirconium also should be removed. The solubility of iron in zirconium is small, so that it is easily precipitated. Parts around the precipitations are chemically active and easily chemically attacked. Particularly, when stresses are applied on the zirconium, the stresses concentrate on the precipitations in the grain boundaries, and cracking takes place from those parts. Therefore, reduction of the precipitations by decreasing an amount of iron contained in the zirconium liner of a nuclear fuel cladding results in decreasing the number of sites which are starting points of cracking. Namely, the reduction in the number of the precipitations has the effect that the liner is not easily chemically attacked by iodine of a corrosive fission product, whereby the prevention of the stress corrosion cracking of the fuel cladding is further improved.
Further, at present, there is a plan that burning of nuclear fuel is extended. The extension of the burning causes corrosive fission products such as iodine to increase. Accordingly, from the point of view of improvement of reliability, the amount of iron contained in the zirconium liner should be reduced.